The primary weapon carried by most soldiers into battle in Iraq and Afghanistan performed the worst in a recent series of tests designed to see how it stacked up against three other top carbines in sandy environments.

After firing 6,000 rounds through ten M4s in a dust chamber at the Army's Aberdeen test center in Maryland this fall, the weapons experienced a total of 863 minor stoppages and 19 that would have required the armorer to fix the problem. Stacked up against the M4 during the side-by-side tests were two other weapons popular with special operations forces, including the Heckler and Koch 416 and the FN USA Special Operations Combat Assault Rifle, or Mk16.

Another carbine involved in the tests that had been rejected by the Army two years ago, the H&K XM8, came out the winner, with a total of 116 minor stoppages and 11 major ones. The Mk16 experienced a total of 226 stoppages, the 416 had 233.

The Army was quick to point out that even with 863 minor stoppages -- termed "class one" stoppages which require 10 seconds or less to clear and "class two" stoppages which require more than ten seconds to clear -- the M4 functioned well, with over 98 percent of the 60,000 total rounds firing without a problem.

"The M4 carbine is a world-class weapon," said Brig. Gen. Mark Brown, the Army's top equipment buyer, in a Dec. 17 briefing at the Pentagon. Soldiers "have high confidence in that weapon, and that high confidence level is justified, in our view, as a result of all test data and all investigations we have made."

Though Army testers and engineers are still evaluating the data, officials with the Army's Infantry Center based in Fort Benning, Ga., said they planned to issue new requirements for the standard-issue carbine in about 18 months that could include a wholesale replacement of the M4. But the Army has been resistant to replace the M4, which has been in the Army inventory for over 18 years, until there's enough of a performance leap to justify buying a new carbine.

"We know there are some pretty exciting things on the horizon with technology ... so maybe what we do is stick with the M4 for now and let technologies mature enough that we can spin them into a new carbine," said Col. Robert Radcliffe, director of combat development at the Army's Infantry Center. "It's just not ready yet. But it can be ready relatively rapidly."

That's not good enough for some on Capitol Hill who've pushed hard for the so-called "extreme dust test" since last spring. Oklahoma Republican Senator Tom Coburn placed a hold on the nomination of Army Secretary Pete Geren earlier this year to force the Army to take another look at the M4 and its reliability.

In an April 12 letter to the still unconfirmed Geren, Coburn wrote that "considering the long standing reliability and lethality problems with the M16 design, of which the M4 is based, I am afraid that our troops in combat might not have the best weapon." He insisted the Army conduct a side-by-side test to verify his contention that more reliable designs existed and could be fielded soon.

Despite the 98 percent reliability argument now being pushed by the Army, one congressional staffer familiar with the extreme dust tests is skeptical of the service's conclusions.

"This isn't brain surgery -- a rifle needs to do three things: shoot when you pull the trigger, put bullets where you aim them and deliver enough energy to stop what's attacking you," the staffer told Military.com in an email. "If the M4 can't be depended on to shoot then everything else is irrelevant."

The staffer offered a different perspective of how to view the Army's result. If you look at the numbers, he reasoned, the M4's 882 total stoppages averages out to a jam every 68 rounds. There are about 30 rounds per magazine in the M4.

By comparison, the XM8 jammed once every 472 rounds, the Mk16 every 265 rounds and the 416 every 257 rounds. Army officials contend soldiers rarely fire more than 140 rounds in an engagement.

"These results are stunning, and frankly they are significantly more dramatic than most weapons experts expected," the staffer said.

Army officials say the staffer's comparison is "misleading" since the extreme dust test did not represent a typical combat environment and did not include the regular weapons cleaning soldiers typically perform in the field.

So the Army is sticking by the M4 and has recently signed another contract with manufacturer Colt Defense to outfit several more brigade combat teams with the compact weapon. Service officials say feedback from the field on the M4 has been universally positive -- except for some grumbling about the stopping power of its 5.56mm round. And as long as soldiers take the time to clean their weapons properly, even the "extreme" dust testing showed the weapon performed as advertised.

"The force will tell you the weapon system is reliable, they're confident in it, they understand that the key to making that weapon system effective on the battlefield and killing the enemy is a solid maintenance program and, just as important, is a marksmanship program," said Sgt. Maj. Tom Coleman, sergeant major for PEO Soldier and the Natick Soldier Systems Center. "So, you can't start talking about a weapon system without bringing in all the other pieces that come into play."

That's not enough for some who say the technology is out there to field a better, more reliable rifle to troops in contact now.

"It's time to stop making excuses and just conduct a competition for a new weapon," the congressional staffer said.